The invention relates to a light source, in particular incandescent lamp, with a bulb, a filament arranged in the bulb, and a hearing device for the filament, the filament emitting both visible light and heat radiation. Furthermore, the invention relates to a method for producing a light source of the above mentioned type.
Light sources of the described type have been known from practice for a long time, and they exist in a large variety of designs and sizes. In this connection, for example, incandescent lamps are known as electrical light sources, in which it is common to bring a tungsten filament by electrical Joule heat to a highest possible temperature. In this process, a temperature radiation is generated. The light yield of incandescent filaments considerably increases as the temperature rises. Besides that, also so-called nonthermal sources of radiation are known, for example, discharge lamps, such as inert gas-, mercury-, sodium-, and metal halide discharge lamps in high-pressure and low-pressure designs.
All so far known, electrically operated types of light sources have the disadvantage that they are very inefficient with respect to converting electric power to visible light output. The conversion barely exceeds 30%. The largest portion of the consumed electric power is an uneconomical dissipation primarily in the form of heat.
A possibility of increasing the efficiency of known light sources consists in that the heat radiated from the filament or glow wire, is reflected from the inner side of the bulb back to the filament or glow wire. As a result, the filament or glow wire undergoes a kind of backheating. This results in that after reaching the same filament temperature, less electric power will be needed than during a heating without reflection. The visible light output, which is transmitted through the bulb, remains in this instance the same. In the ideal case, only that electric power will be needed, which corresponds to the visible, emitted light output and to the thermal dissipation, which is absorbed by the bulb. Thus, the conversion efficiency is improved by the portion of the reflected heat radiation. Theoretically, it would be possible to increase with that the conversion efficiency to as much as 75% or 140 lumens/watt, if one took as a basis the standard thermal dissipation of tungsten lamps of about 25%, and if one neglected the radiation absorption of a mirror coating on the inner side of the bulb. In this connection, for example, dielectric mirror coatings have an absorption of typically 0.1%.
In the case of a mirror coating on the inner side of the bulb with a reflection power of, for example, 99.9%, statistically, every one thousandth photon in the material of the mirror coating will be absorbed. In the case of a reflection of the radiation into the bulb, the photon flux may therefore undergo only 1000 reflections on the inner side of the bulb, until it its totally absorbed in the bulb.
The known filaments present a problem in that, for example, the known spiral form of the filaments or glow wires permits only a very slight absorption of the reflected heat radiation, since the largest portion of the heat radiation is reflected past the thin spiral wire. Thus, an effective absorption or backheating is not possible in the case of conventional filaments or glow wires. Consequently, a high conversion efficiency is not realizable with conventional light sources.
It is therefore an object of the present invention to describe a light source of the initially described type as well as a method for producing such a light source, wherein a high conversion efficiency is achieved with simple means.